1. Field Of The Invention
This invention relates to a process for producing aluminum by electrolyzing aluminum oxide dissolved in a molten salt bath.
2. Description Of The Prior Art
The term "production units of the electrode and electric power", as used herein denotes the amounts of the electrode and electric power required for producing a ton of aluminum as a final product, this value can be used as a basis for calculating the production cost.
In the early stages of the development of Soderberg anodes containing vertical contact studs, an anode paste having a composition that did not form a soup-like or liquid layer was used, and the anode was operated to form an upper layer of unbaked paste which did not clog holes left by the pulling out of the contact studs and a lower layer of baked paste, as disclosed at page 2, left column, line 35 to right column, line 1 of Japanese Patent Publication No. 4738/51. Since this method (hereafter referred to as the first conventional method) must be operated using these two layers, the surface temperature of the unbaked layer must be maintained at more than about 200.degree. C. This causes the volatilization of large amounts of volatile components from the unbaked layer, and results in a reduced anode density. Hence, the anode suffers from a low density and inferior mechanical properties such as flexural strength or compressive strength and inferior electrical characteristics such as specific electric resistance. This in turn leads to various troubles with the anode such as carbon drop-out or very high production units of the electrode and electric power. Furthermore, this method is operationally complex in that it is necessary to pull out the contact studs immediately before their lower ends enter the electrolytic bath as a result of anode consumption, pour a predetermined amount of the paste into the holes left after pulling out the contact studs, and again place the studs in position at a higher level than the original level.
In an attempt to avoid such operational complexities, an improved anode operating method (hereafter referred to as the second conventional method) was suggested in Japanese Patent Publication No. 5155/53, in which an anode paste having a composition that will form a soup-like or liquid layer was used as a material for the anode, and the anode was operated so as to form an upper layer composed of a soup-like or liquid paste which was to flow into the holes left after pulling out the contact studs, an interlayer composed of an unbaked paste which, for a time after the pulling out of the contact studs from the anode did not clog the holes by its own collapsing, and a lower baked layer.
The second conventional method has gained wide commercial use, while the first conventional method is not industrially used at present.
It is true that the second conventional method avoids the operational complexities mentioned above, but according to this method, a fine powdery carbonaceous aggregate in the briquette-shaped anode paste remains in the soup-like or, liquid layer, and the balance of the paste segregates as a gravel-like layer beneath the soup-like or liquid layer, thus changing the composition of the layers of the anode. Accordingly, the anode has a low apparent density, a flexural strength as low as about 70 to 80 Kg/cm.sup.2 and a compressive strength as low as about 250 to 300 Kg/cm.sup.2. These inferior properties cause cracks in the anode when the contact studs are pulled out of the anode, and result in leakage of the fluid paste, drop-out of baked carbon and increased specific electric resistance. Consequently, troubles with the anode occur constantly, and the production units of the electrode and electric power become inferior. Furthermore, this method suffers from other operational complexities. For example, when the contact studs are pulled out of the anode, the soup-like or liquid paste adheres to the surface of the studs having a high temperature, and a carbonaceous material also adheres to the anode casing. Accordingly, extra operations are required to scrape off these materials.